Medical procedures are used to treat a variety of cardiovascular defects, such as cardiac arrhythmias, atrial fibrillation, and other irregularities in the transmission of electrical impulses through the heart. Such medical procedures may involve ablation of the specific tissue that cause or transmit the irregular electrical impulses, e.g., creating lesions or other anatomical effects that disrupt or block electrical pathways through the targeted tissue so as to allow the other tissues to function properly. In the treatment of cardiac arrhythmias for example, a specific area of cardiac tissue having aberrant electrical activity (e.g. focal trigger, slow conduction, excessively rapid repolarization, fractionated electrogram, etc.) may be identified first and then treated.
One example of a type of ablation system involves the delivery of radiofrequency (“RF”) energy to the tissue selected for treatment. RF ablation systems may include a power source or RF generator, and one or more medical devices having at least one ablation element or electrode coupled to the power source. The medical device may be a flexible catheter having a handle at a proximal end and an ablation electrode near a distal end, or may have an array of electrodes which may be configured on one or more carrier arms. Examples of an RF generator and medical ablation catheters having various configurations are illustrated in FIGS. 1-7. One or more sensors may also be provided, such as a temperature sensor, thermocouple, or a sensor for another parameter (such as contact assessment, pressure, etc.), which may be arranged at or near the ablation electrodes. The sensors may be placed near to one or more of the ablating surface of each electrode, or at the interface between the electrode and the tissue to be treated. Such a system may also include one or more external electrodes touching the skin of the patient, which may be called “indifferent” electrodes, also coupled to the power source. After mapping and diagnosing the electrical irregularities, a physician may decide to treat the patient by ablating cardiac tissue. FIG. 8 shows a stylized depiction of an ablation system in use during a medical treatment of the heart of a patient.
It is desirable to enable and ensure continuous contact during an ablation procedure between each ablation element or electrode and the corresponding selected tissue. It is also desirable to maintain a constant electrode temperature during the ablation, at a value sufficiently high to ensure that lesions are created, but not so high that there is a risk of charring and coagulum formation. Feedback controllers that are responsive to a temperature measured at or near the electrodes may be employed to maintain electrode temperature. Sometimes the tissue selected for treatment may be moving, such as for example cardiac tissue of a beating heart, or during the movements associated with respiration.
During such movement, one or more ablation elements may lose contact or be in only intermittent contact with the tissue. When tissue contact is lost, the temperature of the ablation element will ordinarily decrease. In response, it is possible that a feedback controller of an ablation system may temporarily increase power output from the power source. Such a response may be an undesirable reaction to the temperature feedback signal, since the decreased temperature is caused by the loss of firm and continuous tissue contact, and not due to a change in the ablation conditions that would require additional power supplied to the ablation elements
To provide more effective, safe and efficient medical treatments, it is desirable to optimize the ablation system and method of use to avoid excessive local heat which may cause the formation of coagulum. It is also desirable to monitor and recognize the level and character of contact by an ablation element with the corresponding tissue to be treated, and respond accordingly.